# How a roof is cleaned: washing, protective coating, risks and when not to > Washing the tile and protecting it afterwards: what APICER and DECO Proteste say, what they do not, and what to ask about products and guarantee. - Page: https://telhadoseguro.pt/en/roof-cleaning/methods - Language: English - Updated: 2026-11-03 ## Summary - Washing and protecting are two steps and may be quoted separately. - DECO Proteste describes washing from the top down, with the slope; the other way can lift the tiles. - Then anti-moss and a waterproof coating; cleaning can leave the tile more porous. - On old asbestos-cement, do not pressure-wash. There is no independent source on coating durability. Cleaning a tiled roof usually has two parts: **washing** the tile to remove moss, lichen and dirt, and sometimes **applying a protective product** (an anti-moss treatment and a coating that repels water) so the moss takes longer to come back. This guide explains each step, what the sources we found say about the method, and when you should do none of it. No Portuguese standard fixes "the" right method: what follows comes from the APICER and CTCV technical guide and a consumer article from DECO Proteste, and we say where each one stops. *Updated 2 November 2026. General information. Every roof is different: only someone who sees it can say what suits it.* ## The essentials - **First, know what roof you have.** On old asbestos-cement sheets, **do not pressure-wash or scrub**. See the [asbestos guide](https://telhadoseguro.pt/en/guides/asbestos-roof-law-removal). - **Washing and protecting are two different steps** and may be quoted separately. - **DECO Proteste** describes washing with a high-pressure machine, preferably with a brush, **from the top down, in the direction of the slope**. The other way, it can lift the tiles. - **After washing**, the same article recommends an anti-moss treatment applied to a dry roof and, to stop moss returning, a waterproof coating. It warns that cleaning can leave the tile more porous. - **If tiles are broken or displaced, or the battens have given way**, cleaning is not enough: it is repair. See the [guide to leaks](https://telhadoseguro.pt/en/guides/roof-leak-causes). - **Climbing on a roof is work at height.** Leave it to people with fall protection. ## When to clean (and when it is not enough) The APICER and CTCV guide, 2nd edition (2018), says roof faults come mostly from a **total lack of maintenance** and recommends a **yearly routine of inspection and cleaning**. It also describes parasitic vegetation: it can block drainage, move and break tiles and hold water, especially at eaves and in gutters. The guide separates two levels: | Situation | What the guide says | |---|---| | Little vegetation, not tightly attached, no displaced tiles | Maintenance work: clear the vegetation, **practically without lifting the tile** | | Extensive, tightly attached vegetation, or tiles and flashings already displaced or broken | Work that means **lifting the tile**, possibly washing it to put it back, and the unavoidable replacement of some tiles | In practice: surface moss calls for cleaning; moving tiles, overflowing gutters and water stains inside call for someone who **looks at the whole roof** before quoting only a wash. ## Step 1: washing the tile What DECO Proteste (3 February 2022) describes for a ceramic or slate tile roof: 1. **High-pressure machine**, preferably with a brush. A hand brush also works but takes longer. 2. **From the top down, following the slope.** The article warns that the other way the tiles can lift. 3. **A scraper** for what stays stuck. 4. **Block the gutter outlet** towards the downpipe so moss does not clog it. What the article does **not** say: what pressure to use on a roof. The pressure advice it gives is for patios and terraces. So ask the firm what pressure it uses and on what kind of tile. The APICER and CTCV guide only says the tile should stand up well to the **cleaning agents** used in maintenance; it gives no values. ## Step 2: anti-moss and protective coating After the roof dries, DECO Proteste describes an **anti-moss product** (applied with an electric spray gun or sprayer, enough to soak into the pores of the tile) and, to prevent new growth, a **waterproof protection** (applied with a brush), sold in several colours. It says that cleaning ceramic or slate tile can leave it **very porous**, which is why it recommends the protection. This is a consumer article, not a standard. **We found no independent source** on how long a protective coating lasts, or how it behaves on each tile type. So before you accept: - ask for the **product name** and the maker's **data sheet** (and safety data sheet); - ask **how many coats**, **what drying time** and **what weather** it is applied in; - ask **where the run-off goes**: where the wash water and leftover product end up, so it does not fall on a vegetable patch, a well or a neighbour's street; - ask in writing **what the guarantee covers** and for how long. A guarantee of "X years" that does not say what it covers is worth little. Our guide to the [warranty on roof works](https://telhadoseguro.pt/en/guides/roof-works-warranty) explains what the law says. ## What not to do - **Pressure-wash old asbestos-cement.** The ACT, the labour authority, warns that any handling can release asbestos fibres. - **Climb on the roof without protection.** Wet tile, moss and a slope are a fall waiting to happen. See the guide on [hiring a cleaning firm](https://telhadoseguro.pt/en/roof-cleaning/contractor). - **Clean to hide a problem.** A clean tile that is broken still lets water in. - **Use anti-moss or wash water without thinking where it goes.** ## Roofs that need more care - **Slate or very old tile:** the DECO Proteste article mentions the risk of washing making it porous. Ask the firm to explain what it will do in this case. - **A roof in poor condition:** the APICER and CTCV guide asks for a structured assessment before choosing a strategy. Start with the [checklist before winter](https://telhadoseguro.pt/en/guides/roof-checklist-before-winter). - **Sandwich panels and sheets:** we found no source on how to clean them. Ask the maker. ## What we could not confirm - **A "safe" pressure for roofs.** None of the sources gives one. - **Durability and behaviour of a protective coating** on ceramic tile. We only found DECO Proteste's recommendation, with no time spans. - **Portuguese standards** for washing and protecting roofs. ## Sources (read on 2 November 2026) - APICER and CTCV, [Guia para Projeto e Aplicação de Telhas Cerâmicas](https://www.apicer.pt/apicer/media/5c06bd0e8d436.pdf), 2nd edition, 2018 (section 4.3.10; section 7.4.2, page 95; case 6 "parasitic vegetation", page 102). In Portuguese. - DECO Proteste, [Máquinas de limpeza de alta pressão: dicas para trabalhos de manutenção da casa](https://www.deco.proteste.pt/casa-energia/manutencao-casa/dicas/maquinas-limpeza-alta-pressao-dicas-trabalhos-manutencao-casa), 3 February 2022. In Portuguese. - Our guide to [asbestos on roofs](https://telhadoseguro.pt/en/guides/asbestos-roof-law-removal), which cites the ACT and the APA. ## FAQ ### What pressure should be used on a roof? None of the sources we read gives a pressure for roofs. Ask the firm what pressure it uses and on what kind of tile. ### Does a protective coating stop moss coming back? DECO Proteste recommends it to prevent new growth. We found no independent source on how long it lasts. Ask for the data sheet and what the guarantee covers. ### Can I pressure-wash an asbestos-cement roof? No. The ACT warns that any handling can release asbestos fibres. --- Source: https://telhadoseguro.pt/en/roof-cleaning/methods. Independent information, not legal or tax advice.